Percolation and Nanocomposites
In a conductive network, the conductivity can be guided by percolation theory. Mathematically, percolation can be described as the following: in an infinite network, there is a path of connected points of infinite length “through” the network. With increasing population of the conducting points, there exists a critical number above which at least one conducting pathway forms within the network. The critical point is called percolation threshold which is a particle-size and geometry-dependent parameter for composites. Generally, smaller particles allow for a lower percolation threshold, due to the fact that average particle distance decreases with particle size, given the same volume ratio, which helps to build the connection pathway of particles through the whole composite. Particle shape is another important factor. It has been shown, both theoretically and experimentally, that percolation threshold can be significantly decreased by using high aspect ratio particles.
Previously fillers with different compositions, surface lubrications, and shapes were used for electrically conductive composites, utilizing fillers such as metal beads, metal fibers, metal flakes and metal coated flakes. Fillers with high aspect ratio were also used in thermally conductive composites.
Heat transfer in composites is similar to that of electrical conduction except that heat is transported by phonons instead of electrons. Nevertheless, similar to electrical conductivity, any given material has a resistivity against heat flow, therefore, the network rule of electrical conductivity applies to thermal conductivity as well.
Applications of Composites
Composites have been and could be applied in several important areas. One area is electronics packaging. Over the past decade, as on chip interconnect dimensions have been reduced, power to drive the more resistive circuits has become more and more problematic. New packaging solutions that do not require liquid cooling are in demand for today's new chips and circuits. The successful development of composites containing high thermal conductivity fillers may ensure that advanced packaging technology is capable of facing the challenges that may be presented by current and future generations of chips and circuits.
Another area where composites could be applied is for use as temporary bonding materials for wafer processing. To accommodate the ever increasing demand for smaller IC devices in cell phones, music players, cameras etc, it has become common to grind the fully processed device wafers to be thinner. After the device wafer is processed on the front side, it is coated with a temporary bonding adhesive on the front and further processed on the back (thinning, etching, metallization, etc.). The device wafer is then detached from the temporary bonding materials. The wafer thinning is often done using aggressive methods that generate a substantial amount of heat, which inevitably raises the wafer temperature, sometimes to as high as 300° C. Better heat dissipation may reduce the device temperature and/or allow for more aggressive thinning methods to reduce processing times. Incorporation of thermal conductive fillers could help to increase the thermal conductivity of the temporary bonding materials. Using traditional conductive fillers to effectively increase the thermal conductivity of the composite typically requires high levels of loading, which significantly decrease the bonding strength of the composites. Therefore, it would be useful to develop a composite with low filler loading and high thermal conductivity for better temporary bonding materials.
A third area where composites can be used is for polymers that are electrically conducting, which can be used in many applications including conducting pastes or adhesives, charge dissipation materials, transparent conductors, and electromagnetic interference shielding (EMI shielding).
Conducting paste or adhesives are often made by mixing conductive filler into adhesive materials and have been widely used in the advanced packaging industry (such as die attach materials to provide mechanical adhesion and electrical conductivity between dies and printed circuit board; or lead-free solder materials to provide electrical interaction between devices) in forms of paste, gel or tape. High electrical conductivity often requires high filler loading. Common fillers used include metal flakes, metal fibers, carbon black or carbon fiber. However, adhesion strength and/or mechanical properties often suffer as a result of high filler loading. Therefore, it is desirable to achieve electrical conductivity using low filler loading.
Transparent conductor materials are required to have a combination of optical transparency and electrical conductivity. Their application can be found in flat panel displays, solar cells, smart windows, photovoltaics, EL lighting and a variety of other optical and electronic applications, where they can deliver or collect electrons from the active part of the device while allowing visible photons to pass through without a significant loss. Transparent conductors for commercial scale need to be processed easily and cost effectively. Indium Tin Oxide (ITO) is the most widely used transparent conductor due to its superior combination of transparency and conductivity. In some applications, fluorine-doped indium oxide (FTO) is used as an alternative to ITO. However, ITO, as well as FTO, is expensive due to the short supply of indium. Moreover, ITO is far from ideal for many of the fastest growing application sectors in which transparent conductors are used. For instance, the inherit brittleness of ITO constrains its application in touch screen displays since it cracks easily. Other transparent electrical conductive film, such as transparent carbon nanotube sheet, while is strong and flexible, is more costly and has not fully entered industrial applications.
Electric charges, induced by contact, pressure, or heat, build up on an object with low electrical conductivity. Rapid discharge of the static charge build up can generate a large electric current or an electrical spark, which may be extremely harmful to electronic devices, around flammable and ignitable materials, and in space exploration. Increasing the surface conductivity of the materials helps to reduce the charge build up and dissipates static charge to the ground constantly.
Electrically conductive materials are also commonly used for EMI shielding. EMI exists when an electromagnetic disturbance induces undesirable voltages or currents that adversely influence the performance of electronics or electrical devices. EMI in radio communications has also been called radio frequency interference (RFI). Currently, certain frequency ranges are prohibited or rigorously regulated by the government and/or the military. The greatest concern about EMI besides communications is its effect on electronic devices such as onboard sensor systems, pacemakers, electrosurgical units and personal computers. Electromagnetic interference works in different ways to degrade the performance of an electronic device. The most common form of interference is the electrical current generated in an electrical circuit, when it is hit by an electromagnetic disturbance. Depending upon the magnitude of the electrical disturbance, the induced electrical current can either corrupt a low level signal or override and eventually destroy the circuit.
Materials with high electrical conductivity often have high thermal conductivity as well. However, in an application such as an electric-thermal heating unit, materials with high electrical conductivity and low thermal conductivity are needed. Materials are needed in the form of bulk, fibers, and films. Some other properties, such as transparency, thermal fatigue tolerance, and toughness need to be optimized as well.
Example material compositions and processing methods disclosed herein provide domain segregation of blends of polymers or polymer and small molecule compounds, and/or block copolymers useful to generate a thermally and/or electrically conductive pathway composed of thermal and/or electrical conducting fillers.
Composite materials conduct heat and/or electricity through pathways constructed through domain segregation of polymer blends and/or block copolymers. The matrix polymers provide mechanical strength to the composite and necessary binding properties. They also provide confinement to the second phase materials, preferably with low melting points or low softening temperatures. Thermal/electrical conductive fillers are purposely and preferentially dispersed in the second phase material domains. At the operating temperature, the second phase materials tend to swell, melt and/or flow and the conductive fillers, such as highly mobile particles can align to form an effective thermal/electrical conducting pathway.
An example non limiting implementation provides a multi-component material for thermal conduction, comprising, a first component comprising a matrix polymer, a second component comprising a low melting point material immiscible with the first component, and a third component comprising a filler material with higher thermal conductivity than the first and second components. Wherein the third component is dispersed into the second component and the second component is dispersed within the polymer matrix.
An example non limiting implementation provides a process of making a multi-component fluid for use in manufacturing thermally enhanced layers. This multi-component fluid comprising a polymeric matrix material, a low melting point material and filler particles. The process comprising capping filler particles with capping agents immiscible with the polymeric matrix material and miscible in the low melting point material, dispersing capped filler particles in the low melting point material and creating a mixture by combining the polymeric matrix and the low melting point material including the dispersed capped filler particles.
An example non limiting implementation provides a product formed by the process of making a multi-component fluid. This multi-component fluid comprising a polymeric matrix material, a low melting point material and filler particles. The process comprising capping filler particles with capping agents immiscible with the polymeric matrix material and miscible in the low melting point material, dispersing capped filler particles in the low melting point material and creating a mixture by combining the polymeric matrix and the low melting point material including the dispersed capped filler particles.
An example non limiting implementation provides a process for assembling a system having enhanced thermal conductivity. This system comprising an integrated circuit, a heat sink, and a multi-component composite material. The process comprising forming a multi-component composite thermal conductor between the integrated circuit and the heat sink. The multi-component thermal conductor comprises a matrix material and a second phase material with high thermal conductive filler material.
An example non limiting implementation provides a multi-component electrical conductor with a first component that is a matrix polymer, a second component that is a low melting point material immiscible with the first component; and a third component that is a filler material with higher electrical conductivity than the first and second components. The third component is dispersed into the second component and the second component is dispersed within the polymer matrix. The third component provides enhanced electrical conductivity to the multi-component electrical conductor. In addition an example multi-component electrical conductor may comprise an optically transparent first component and an optically transparent second phase material and a third component that is a filler material with higher electrical conductivity than the first and second components. The refractive indices of the polymer matrix and the second phase material may be similar to each other to minimize internal reflection and maximize transparency.
One exemplary non-limiting illustrative embodiment provides a composite composition made up of multiple components. The multiple component system include, at least two components, preferably three components, and more components in certain applications. This multiple component system includes polymers and fillers to improve the thermal and/or electrical conductivity of the matrix material in at least one dimension.
One exemplary non-limiting illustrative embodiment provides matrix material that is thermoplastic. Examples include, but are not limited to, poly(acrylonitrile-butadiene-styrene) (ABS), poly(methyl methacrylate) (PMMA), celluloid, cellulose acetate, poly(ethylene-vinyl acetate) (EVA), poly(ethylene vinyl alcohol) (EVOH), fluoroplastics, polyacrylates (Acrylic), polyacrylonitrile (PAN), polyamide (PA or Nylon), polyamide-imide (PAI), polyaryletherketone (PAEK), polybutadiene (PBD), polybutylene (PB), polybutylene terephthalate (PBT), polycaprolactone (PCL), polychlorotrifluoroethylene (PCTFE), polyethylene terephthalate (PET), polycyclohexylene dimethylene terephthalate (PCT), polycarbonate (PC), polyhydroxyalkanoates (PHAs), polyketone (PK), polyester, polyethylene (PE), polyetheretherketone (PEEK), polyetherketoneketone (PEKK), polyetherimide (PEI), polyethersulfone (PES), polyethylenechlorinates (PEC), polyimide (PI), polylactic acid (PLA), polymethylpentene (PMP), polyphenylene oxide (PPO), polyphenylene sulfide (PPS), polyphthalamide (PPA), polypropylene (PP), polystyrene (PS), polysulfone (PSU), polytrimethylene terephthalate (PTT), polyurethane (PU), polyvinyl acetate (PVA), polyvinyl chloride (PVC), polyvinylidene chloride (PVDC), poly(styrene-acrylonitrile) (SAN), etc.
One exemplary non-limiting illustrative embodiment provides matrix material that is a rubber. Examples include, but are not limited to, silicon rubber, fluorinated silicone rubber, natural rubber, vulcanized rubber, nitrile rubber, styrene butadiene rubber, ethylene propylene diene rubber, neoprene, polyisoprene, polybutadiene, butyl rubber, urethane rubber, hypalon polyethylene, polyacrylate rubber, epichlorohydrin, fluoro carbon rubber, hydrogenate nitrile, etc.
One exemplary non-limiting illustrative embodiment provides matrix material that is thermosetting polymer. Examples include, but are not limited to, epoxy resin, phenolic resin, unsaturated polyester, melamine resin, urea-formaldehyde, etc.
One exemplary non-limiting illustrative embodiment provides having second phase material being a polymer material with melting point, or softening point, or flow point lower than the operating temperature. Examples of a second phase material include, but are not limited to, poly(acrylonitrile-butadiene-styrene) (ABS), poly(methyl methacrylate) (PMMA), celluloid, cellulose acetate, poly(ethylene-vinyl acetate) (EVA), poly(ethylene vinyl alcohol) (EVOH), fluoroplastics, polyacrylates (Acrylic), polyacrylonitrile (PAN), polyamide (PA or Nylon), polyamide-imide (PAI), polyaryletherketone (PAEK), polybutadiene (PBD), polybutylene (PB), polybutylene terephthalate (PBT), polycaprolactone (PCL), polychlorotrifluoroethylene (PCTFE), polyethylene terephthalate (PET), polycyclohexylene dimethylene terephthalate (PCT), polycarbonate (PC), polyhydroxyalkanoates (PHAs), polyketone (PK), polyester, polyethylene (PE), polyetheretherketone (PEEK), polyetherketoneketone (PEKK), polyetherimide (PEI), polyethersulfone (PES), polyethylenechlorinates (PEC), polyimide (PI), polylactic acid (PLA), polymethylpentene (PMP), polyphenylene oxide (PPO), polyphenylene sulfide (PPS), polyphthalamide (PPA), polypropylene (PP), polystyrene (PS), polysulfone (PSU), polytrimethylene terephthalate (PTT), polyurethane (PU), polyvinyl acetate (PVA), polyvinyl chloride (PVC), polyvinylidene chloride (PVDC), poly(styrene-acrylonitrile) (SAN), etc.
One exemplary non-limiting illustrative embodiment provides second phase material that is a small organic or inorganic molecule or an oligomer with melting point lower than the operating temperature. Examples include, but are not limited to, paraffin (CnH2n+2), fatty acids (CH3(CH2)2nCOOH), alkylamines (CH3(CH2)2nNH2), salt hydrates (MnH2O), (where M refers to a metal), etc.
One exemplary non-limiting illustrative embodiment provides having the same material as the polymer matrix and the second phase polymer. This multiple component system only has two components with component A being the matrix and component C being the conductive filler.
One exemplary non-limiting illustrative embodiment provides fillers with low aspect ratio. Examples include, but are not limited to, C, Si, Ge, Ag, Au, Cu, Ni, Pt, Pd, Fe, Pb, Al, Zn, Co, Dy, Gd, CuCl, CuBr, CuI, AgCl, AgBr, AgI, Ag2S, Al2O3, Ga2O3, In2O3, FeO, Fe2O3, Fe3O4, TiO2, MgO, Eu2O3, CrO2, CaO, MgO, ZnO, MgxZn1-xO, SiO2, Cu2O, Zr2O3, ZrO2, SnO2, ZnS, HgS, Fe2S, Cu2S, CuIn2S2, MoS2, In2S3, Bi2S3, GaP, GaAs, GaSb, InP, InAs, InxGa1-xAs, SiC, Si1-xGex, CaF2, YF3, YSi2, GaInP2, Cd3P2, CuIn2Se2, In2Se3, HgI2, PbI2, ZnSe, CdS, CdSe, CdTe, HgTe, PbS, BN, AlN, GaN, InN, AlxGa1-xN, Si3N4, ZrN, Y2O3, HfO2, Sc2O3, etc.
One exemplary non-limiting illustrative embodiment provides fillers with high aspect ratio. Examples include, but are not limited to, C, Si, Ge, Ag, Au, Cu, Ni, Pt, Pd, Fe, Pb, Al, Zn, Co, Dy, Gd, CuCl, CuBr, CuI, AgCl, AgBr, AgI, Ag2S, Al2O3, Ga2O3, In2O3, FeO, Fe2O3, Fe3O4, TiO2, MgO, Eu2O3, CrO2, CaO, MgO, ZnO, MgxZn1-xO, SiO2, Cu2O, Zr2O3, ZrO2, SnO2, ZnS, HgS, Fe2S, Cu2S, CuIn2S2, MoS2, In253, Bi2S3, GaP GaAs, GaSb, InP, InAs, InxGa1-xAs, SiC, Si1-xGex, CaF2, YF3, YSi2, GaInP2, Cd3P2, CuIn2Se2, In2Se3, HgI2, PbI2, ZnSe, CdS, CdSe, CdTe, HgTe, PbS, BN, AlN, GaN, InN, AlxGa1-xN, Si3N4, ZrN, Y2O3, HfO2, Sc2O3, layered silicate clays, talc, layered perovskites, etc.
Another exemplary non-limiting illustrative embodiment provides a multiple component composite system with two polymers and two fillers. The second filler can either be a conductive filler to further enhance the thermal and/or electrical conductivity of the composite or mechanical reinforcing filler which provides extra physical strength to the composites. Example of the second filler include, but are not limited to, alkyltrimethylsilane capped silica colloids preferentially entering the matrix material (component A) as thermally conductive filler, alkylphosphoric acid capped carbon nanotubes preferentially entering the matrix material (component A) as thermal and/or electrical conductive filler and mechanical reinforcing filler.
One exemplary non-limiting illustrative embodiment provides methods for applying the composite. The methods include, but are not limited to, curing, polymerization, laminating, extrusion, injection molding, mold casting, spin coating, dip coating, brushing, spraying, printing, etc.
One exemplary non-limiting illustrative embodiment provides having a high aspect ratio filler chemically bonded with the low melting point polymer that is in-situ polymerized. The chemical bonds promote the alignment of the high aspect ratio fillers as the low melting point polymer melt and flow at the working temperature.